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Pacific Ocean

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PACIFIC OCEAN - the ancient world was ignorant of the existence of the vast expanse of water now known as the Pacific Ocean. In Ptolemy's map of the world, constructed in the 2d century of our era (see MAP, vol. xv. Pl. VIT.), this fact is clearly brought out, for the only space which might possibly represent the Pacific is the Magnus Sinus, ; a sea so limited in extent, and represented in such a position, that it probably stands for the Gulf of Siam in the Indian Ocean.

Vague reports of a great ocean lying beyond China were current in Europe as early as the period of Arabian supremacy in learning. Indeed an Arab merchant named Sulaiman, who visited China in the 9th century, declared that he had sailed upon it. But for several hundred years the reports continued so uncertain, and were so loaded with the wild extravagance of travellers' tales of the period, that it is difficult to get at the facts from which they probably took their origin. During the 13th and 14th centuries Marco Polo arid his successors travelled far to the East and came to an ocean of the extent of which they were ignorant, but they partially explored its western coasts. The East was the region towards which all the commerce and enterprise of the Middle Ages tended, and it was the hope of finding a safer and shorter sea route to India that led the Spanish court in 1492 to furnish Columbus with a fleet for the exploration of the Western Ocean. Although convinced of the spherical form of the earth, he greatly under-rated its size, and, accepting the popular estimate of the great breadth of the Asiatic continent, he set out on his voyage confident of soon reaching " the Indies." The glowing descriptions of his discoveries in that strange new world of the West that' rose up before him to bar his advance immediately attracted the attention of adventurous Spanish mariners. Headed by Columbus himself, they cruised intrepidly amongst the Caribbean Islands, still lured by the hope of discovering some western passage to the coveted East. Columbus found that what he at first considered a labyrinthine archipelago was a continent of vast extent, but not Asia, and he died without knowing what lay beyond. Spain and Portugal were the rival maritime powers at that time, and both took up the search for new countries with great ardour. Pope Alexander VI., in 1493, fearing that the two nations would quarrel over their colonies, assigned all the new lands that might be discovered west of the Azores to Spain, and all east of those islands to Portugal. The Portuguese accepting the gift followed Vasco da Gama in opening up the road to India by the Cape of Good Hope, and pushed forward their trading and piratical excursions into the west Pacific far beyond the Spice Islands. The Spaniards confined themselves to the New World, visiting, naming, and plundering the West India Islands and the headlands of Central America. On the 29th of September 1513 Vasco Nunez de Balbao, the leader of a Spanish party exploring the Isthmus of Panama, saw, from the summit of a mountain, a vast ocean stretching to the west - the very ocean of whose existence Columbus was certain, and which he had so long tried vainly to discover. Because he first saw it on Michaelmas day, Balbao named it the G'olfo de San Miguel. Magellan, following the east coast of America farther to the south than any previous explorer, sailed on, in spite of terrific storms, until he found the strait which now bears his name, and, steering carefully through it, on the 27th of November 1520 he swept into the calm waters of that new sea on which lie was the first to sail, and which he named the Mar Pacitico.

The victories of Cortez in Mexico about the same date opened the way for the exploration of the west coast of America, where Pizarro's conquest of Peru in 1526 gave the Spaniards a firm footing. From this time an intermittent trade sprang up between Europe and the Pacific through Magellan Strait, and latterly round Cape Horn. Before long English fleets, attracted more by the prospects of plundering Spanish galleons than of discovering new territories, found their way into the Pacific. Sir Francis Drake, like Balbao, saw the ocean from the Isthmus of Panama. He entered the Pacific in September 1577, being the first Englishman to sail upon it ; some months later he sailed across it to the Moluccas. Alvaro de Mardana, who preceded him, had discovered the Solomon Islands in 1567.

Tasman, Roggewein, Dampier, and other explorers of the 17th century discovered Australia, New Zealand, Tasmania, and many smaller groups of islands. During the 18th century the voyages of Anson, Bass, Behring, the two Bougainvilles, Broughton, Byron, Cook, La Perouse, and many more practically completed the geographical exploration of the Pacific Ocean. In the beginning of that century the Pacific had a curious fascination for commercial speculators, and the ill-fated Scottish colony founded at Darien in 1698 seemed only to prepare the way for the English South Sea bubble that burst in 1720. All the navigators who explored these seas believed in the existence of a north-west passage between the Atlantic and Pacific, and made attempts to find it ; but its discovery baffled all enterprise until 1850, when Maclure proved that there was such a channel, but that the ice prevented its being of any commercial utility. In the present century D'Entrecasteaux, Krusenstern, Beechy, Fitzroy, and Bennet have taken the lead amongst geographical.explorers in the Pacific, although the ranks contain many names scarcely less worthy of remembrance. Within recent years several purely scientific exploring expeditions and British surveying vessels have examined the Pacific, investigating its depth, the nature and form of the bottom, the temperature of the water at various depths and its density, as well as the marine fauna and flora. Of those expeditions the voyages of the " Challenger," " Gazelle," and " Tuscarora " are the most important) Extent. - The Pacific Ocean2 is bounded on the N. by Behring Strait and the coasts of Russia and Alaska, on the E. by the west coasts of North and South America ; on the S. the imaginary line of the Antarctic Circle divides it from the Antarctic Ocean, while its western boundary is the east coast of Australia, the Malay Archipelago separating it from the Indian Ocean, and the eastern coasts of the Chinese empire. Some modern geographers place the southern limit of the Atlantic, Pacific, and Indian Oceans at the 40th parallel, and name the body of water which surrounds the earth between that latitude and the Antarctic Circle the Southern Ocean.

Although differing from the Atlantic in its general form, being more nearly land-locked to the north, the Pacific resembles it in being open to the south, forming, in fact, a great projection northwards of that vast southern ocean of which the Atlantic is another arm.

The Pacific is the largest expanse of water in the world, covering more than a quarter of its superficies, and comprising fully one-half of its water surface. It extends through 132 degrees of latitude, in other words, it measures 9000 miles from north to south. From east to west its breadth varies from about 40 miles at Behring Strait, where Asia and America come within sight of each other, to 8500 miles between California and China on the Tropic of Cancer, and to more than 10,000 miles on the Equator between Quito and the Moluccas, where the ocean is widest. The area has been variously estimated at from 50,000,000 to 100,000,000 square miles ; but, defining its boundaries as above, Keith Johnston, from careful measurements, estimated it, with probably a near approach to the truth, at 67,810,000 square miles.

Coasts, Seas, ((e. - The coast-line of the Pacific and Indian Oceans, taken together, only amounts to 47,000 miles ; that of the Atlantic alone measures 55,000, the smaller ocean more than making up for its loss extent by its numerous inland seas and inlets of smaller size. Speaking broadly, the eastern boundary of the Pacific is rugged, barren, mountainous, and singularly free from indentations, while its western shores are low, fertile, and deeply indented with gulfs and partially enclosed seas. Behring Strait unites the Arctic Ocean with the Sea of Kamchatka, or Behring Sea, which is bounded on the east by the irregular, low, swampy shores of Alaska, and on the south by the Alaskan peninsula and the Aleutian Islands. Along British North America the coast is rugged, rocky, considerably indented, and, between the parallels of 50° and 60° N. lat., fringed with islands. The largest of these are Vancouver Island in the Gulf of Georgia, Queen Charlotte Island, Prince of Wales Island, and the islands of King George III.'s Archipelago. The Gulf of California runs northwards in the Mexican coast, reaching from 23° to 32° N. lat. It is the one important inlet on the whole west coast of America, - the only others which are worth naming being the Gulf of Panama and the Gulf of Guayaquil. The Mexican shore is low, and contrasts with the coasts to the north and to the south, which are generally steep and rocky, though there are occasional sandy beaches in Peru and Chili. The breadth of the plain between the Rocky Mountains and the sea gradually diminishes towards the south, and the mountain chain of the Andes runs close along the west coast of South America to the very extremity of the continent.

A series of volcanoes, active and extinct, runs round the Pacific, commencing at Cape Horn, passing along the Andes and Rocky Mountains, crossing from the American continent by the Aleutian Islands to Kamchatka, and thence southwards by Japan and the East Indian Archipelago. to New Zealand. Earthquakes are frequent all along this. line.

There are few islands near the American coast north of Patagonia, and these are small and unimportant; but south of the 40th parallel there is a complete change. The end of the continent seems as if it had been shattered ; there are abrupt bays and jagged chasms; archipelagos of small islands rise up in splintered fragments along the shore. The Strait of Magellan forms a tortuous channel between the mainland and the rocky storm-beaten islands of Tierra del Fuego.

The coast-line on the Asiatic side is longer and greatly diversified. In the north the Sea of Okhotsk is cut off from Behring Sea by the peninsula of Kamchatka, from the extremity of which a chain of islands extends to the borders of the Antarctic Ocean. These islands are of all sizes, ranging from small islets to the island continent of Australia. The island chain hangs in loops along the Asiatic coast, each loop including an almost land-locked sea. These partially enclosed seas are more or less completely cut off from the general oceanic circulation, and they consequently differ considerably from the open ocean as regards the temperature of the water, specific gravity, fauna and flora, and nature of the deposits. The Kurile Islands run from Kamchatka to Japan, cutting off the Sea of Okhotsk. The great Japanese Islands, with Saghalien to the north and the Chinese coast on the west, enclose the Sea of .Japan, leaving it in communication with the Sea of Okhotsk by the Channel of Tartary to the north, with the ocean on the west by the Straits of La P6rouse and Sangar, and on the south by the Straits of Corea. The Yellow Sea runs into the Chinese coast, and is divided from the Sea of .Japan by the peninsula of Corea. The China Sea, with the two great gulfs of Tonquin and Siam, is marked off from the Indian Ocean by the peninsula of Malacca - remarkable because it runs in the same direction as the other two peninsulas of the Pacific, Kamchatka and Corea - and the islands of Sumatra and Java, while Borneo and the Philippine -Islands separate it from the Pacific. Between the south coast of China and the north of Australia the East Indian Archipelago cuts up time ocean into a network of small seas and narrow channels. The seas are named the Celebes, the Banda, the Sulu, the Java, the Flores, and the Arafura. The more important of the sea passages between the islands are the Straits and Channel of Formosa, which lead northward from the Pacific to the China Sea ; the Strait of Macassar between Borneo and Celebes ; Molucca Passage between Celebes, the Moluccas, and Jilolo ; and Torres Strait between New Guinea and Australia. The east coast of Australia is, as a rule, steep and rocky ; there are few inlets, and none of them compare in size with the Gulf of Carpentaria on the north coast. Moreton Bay and Port Jackson are two of the best harbours, and as a haven the latter has few equals in the world. The Great Barrier Reef lies off this coast for a length of more than a thousand miles, the distance between it and the shore varying from 60 to 100 miles. Bass Strait separates Australia from Tasmania on the south ; and the two main islands of New Zealand, separated by Cook Strait, lie to the southeast of the continent. The Gulf of Hauraki, the Bay of Plenty, and Pegasus Bay are the chief inlets in these islands.

River-Sptem. - The drainage area of the Pacific Ocean is estimated at 8,660,000 square miles, while that of the Atlantic amounts to more than 19,000,000 ; the chief reason for this disparity is that only half a million square miles of the American continent are drained into the Pacific, the remaining six and a half millions being connected with the Atlantic river-system, and it is estimated that only one-seventh of the area of the Asiatic continent drains into the Pacific Ocean. The huge wall of the Andes . practically reduces the Pacific rivers of South America to the rank of mountain streams; the Biobio and the Maypu iii 1 Chili are the only ones exceeding 100 miles in length, - the former having a course of 180, the latter of 160 miles. The Rocky Mountain chain, which forms the watershed of North America, runs parallel to the Pacific coast at a distance of about 1000 miles, and the Cascade and minor ranges which skirt the shore are broken through in several places to give passage to rivers that are, in some cases, of considerable size. The Colorado rises in the State of that name, at the base of the Rocky Mountains, flows southwest through Utah and Arizona, and falls into the head of the Gulf of California. Its course measures about 1100 miles, and it drains a rugged and barren area of 170,000 square miles. California has only one river, the Sacramento, 420 miles long. The Oregon (or Columbia) is formed by the union of two streams rising in the Rocky Mountains, one in British Columbia, the other in Idaho. It is a swift-flowing river, full of rapids and cataracts, and, though it is only 750 miles long, the area which it drains is greater by one-seventh than that drained by the Colorado. The ebb and flow of the tide are perceptible for a hundred miles from the mouth of the Oregon, and the river is navigable for that distance. The Frazer, which has a length of 600 miles, flows southward through British Columbia from the Rocky Mountains, and enters the sea in the Gulf of Georgia opposite Vancouver Island, carrying off the rainfall of 98,000 square miles. The northern limit of the American mountain chains is marked by the rise of the great river Yukon, which traverses Alaska ; and, after a run of more than 2000 miles, it enters Gehring Sea opposite the island of St Lawrence. Its tributaries have not been fully explored, so the area which they intersect is unknown, but probably it is very large.

The Asiatic division of the Pacific river-system is very much more extensive than the American, and includes many streams of great size and of considerable commercial importance. In the north the Amur is more than 2000 miles long, and it receives many tributaries, which rise on the north in the Stanovoi mountains, and on the west and south on the borders of the great table-land of the Gobi, the central Asiatic desert ; altogether its basin measures nearly 900,000 square miles. The Hoang-ho (Hwang-ho or Whang-ho) and the Yangtzc-keang both rise near the Kuen-lun mountains of Tibet amongst the extensive terraces which form the eastern slope of the great table-land of Central Asia. The Hoang-ho has a length of 2600 miles, and in its course it sweeps in a northerly curve close to the In-Shan mountains ; then, after being crossed repeatedly by the Great Wall of China, it turns sharply to the south, and finally runs due east into the Yellow Sea. The Yangtze-keang follows a southward direction from its source, but ultimately turns to the north-east and enters the Yellow Sea not far from the mouth of the Hoang-ho. It is one of the longest rivers in the world, for, including its windings, it measures 3200 miles from its source to the sea, These two rivers drain more than a million and a quarter square miles ; and it is principally owing to the large amount of suspended matter which they carry down that the sea into which they fall is called the Yellow Sea. The other rivers of importance are the Choo-keaug, the Mekong, and the Menam. The last two run into the Gulf of Siam, after watering the peninsula of Siam and Cochin China. Few rivers enter the Pacific on the cast coast of Australia, and in consequence of the proximity of the mountains to the shore they are short and unimportant.

Atmospheric Pressure and Prevailing Winds. - When the mean atmospheric pressure for the year over the entire • surface of the world is considered, it is found that there are two broad belts of high pressure which encircle the globe, one on each side of the equator. There is a wide area of slowly diminishing pressure between them, including a narrow central baud along which the barometric readings attain a minimum. Two other regions of low pressure surround the poles, and extend to a considerable distance. That around the North Pole is connected with an area of still lower pressure over the North Pacific, and there is another permanent depression, which is even deeper, in the vicinity of Iceland. Atmospheric pressure is the fundamental meteorological phenomenon, and the mean pressure for the year affords a clue to the cause of all such regular and continuous phenomena as trade winds and ocean currents, and to the distribution of temperature. Similarly a study of the isobars at different seasons throws light upon all periodical occurrences in the way of winds and currents.

A low barometer is always accompanied by a high percentage of atmospheric aqueous vapour ; consequently the equatorial belt of continuous low pressure is a region of almost continuous rain, excessive cloud, and constant calm or light variable winds. The effect of a difference in atmospheric pressure being established between two places is to produce a flow of air from the region of high towards that of low pressure, and the winds in their turn largely determine the surface movements or drift currents of the ocean. The region of calms between the north and south trades in the Pacific is both narrower, more irregular, and less clearly marked than the corresponding belt in the Atlantic. In the East Pacific it lies, at all seasons, considerably north of the equator ; but during the southern summer it is found south of the line in the western parts of the ocean, and disappears entirely in the northern summer, as the calms of the Indian Ocean do also. The reason of the southern position of the west end of the calm belt seems to be the simultaneous occurrence of low atmospheric pressure in the interior of Australia and an exceptionally high barometer in Asia. In the southern winter the depression over Asia and the increase of pressure over Australia form an unbroken barometric gradient, and the result is that the calms are replaced by a southerly breeze of great regularity. The region of calms included between the zones of the two trade winds, and towards which they blow, is not the only one with which they are associated ; for the opposite meteorological conditions that characterize the northern border of the north-east trades and the southern margin of the southeast winds produce two fringing bands of calms. These regions are characterized by a high barometer, a sunny sky, and occasionally sudden squalls, - contrasting with the depressed barometer and dull, wet weather of the equatorial region. In January the low atmospheric pressure over the North Pacific produces winds which affect the climatological conditions of the shores in very different ways. At Vancouver Island the prevailing wind is southwest, and consequently the winter on the shores of British Columbia is mild and moist. The opposite coast of Asia is visited during the same season by northerly winds, - north-east in Alaska, north-north-east in Kamchatka, and north-west in Japan ; and, as a result, the weather in these regions in winter is dry and bitterly cold. The West Pacific and the Indian Ocean are the regions of monsoons, - winds that blow as steadily as the trades, but which change their direction with the season. During the periods of transition the steady breeze gives place to variable winds, occasional calms, and sometimes terrific hurricanes. The general direction of the monsoons in the Pacific between April and October is southerly and south-easterly, and from November to April they blow from the north-east, and on nearing the continent of Asia from the north-west. Monsoonal winds are found connected with all continents ; they are produced by the great differences in the temperature and pressure which prevail over the land at different seasons as compared with the adjacent ocean. The monsoons give rise to oceanic currents which flow in the same direction as the wind, and like it run opposite ways during alternate half years. Although the velocity of the wind over the open sea is always greater than that near shore or on land, it was shown by the observations of the "Challenger," in the Pacific and other oceans, that there is no distinct diurnal variation in the wind's force at sea, though very decided periods of maxima and minima were noticed in the vicinity of land (see METEOROLOGY, vol. xvi. p. 125).

Currents. - The system of surface circulation in the Pacific is much more complicated and less clearly defined than that in the Atlantic, as might be expected from the less constant character of the winds. The latter ocean has two wide channels of communication with the Arctic Sea, while, so far as currents are concerned, the Pacific is landlocked to the north - Behring Strait being narrow and shallow ; consequently water enters the Pacific almost entirely from the south, where there is uninterrupted communication with the Antarctic Ocean. There is no direct information as to the movements of ocean water at depths greater than 200 or 300 fathoms ; it is known, however, from indirect evidence, that movements do occur. Although the subject of under-currents at depths less than those just mentioned has been extensively studied, it is only with respect to surface currents that anything very definite is as yet known.

The vast extent of the Pacific Ocean gives full scope for the current-producing action of tides and winds, while the smooth continental boundary on its eastern side, the numerous groups of islands which break its surface, and its indented western coast, combine to modify the direction of the main streams and to produce innumerable minor currents, some permanent, and others varying from time to time in velocity and direction. The chief cause of these currents is believed to be traceable to the direct or indirect action of wind ; but here it is proposed to refer merely to their general geography and physical effects, without discussing the theory of their formation.

A general surface drift of the cold waters of the Antarctic Ocean, having a temperature lower than 40° Fahr. at all seasons, bears north-east towards Cape Horn, where it divides into two branches ; one, the Cape Horn current, passes on into the Atlantic, and the other sweeps northward along the west coast of South America until it strikes the Peruvian shore, which deflects it westward. The cooling effect of this current on the water all along the coast is illustrated very clearly by the abrupt northward turn of the isothermals (see METEOROLOGY, figs. 8 and 9), which is more conspicuous in the chart for the southern winter than in that for the summer. In summer, however, there is a more striking evidence of this current's cooling power to be seen in the arrangement of the isothermals. The northern line of 70° Fahr. reaches as far south as 18° N. lat., and that of 80° makes a short loop from 18° N. to the equator ; but the southern isothermal of 80° does not touch the American coast at all, and that of 70° lies farther from the equator than 30' S. lat., so that the increase of temperature from the south is very gradual ; so much so that at the Galapagos Islands, under the equator, the temperature of the surface water is only 70°, while a few hundred miles to the west it is over 80'. Penguins - essentially Antarctic birds - are found living on the shores of these islands. In consequence of this current, the highest surface temperature at all seasons of the year is found distinctly to the north of the equator in the eastern Pacific.

The Peruvian current forms the southern fork of the great equatorial current, which runs due west. This current is very broad, and divided by a narrow countercurrent flowing in an opposite direction through its centre. The two branches of the equatorial current occupy very approximately the two areas of falling barometer between the north and south belts of high pressure and the central trough of minimum barometric readings. This difference of atmospheric pressure on each side produces the northeast and south-east trade winds, and to these the current probably owes its regularity and constant direction. The counter-current lies in the narrow belt of low barometric pressure to which the trades blow, and probably originates from the banking up of the waters to the westward. Its rate and position consequently vary greatly at different times of the year. The "Challenger," on her cruise between the Sandwich and Society Islands, found these currents to run with considerable force. In the " Narrative" of the cruise (chap. xviii.) the fact is alluded to thus: - " From Hawaii Island to the 10th parallel the direction of the current was westerly, and its average velocity 18 miles per day, ranging from 10 to 23 miles. From the 10th to the 6th parallel the direction was easterly, and its average velocity 31 miles per day, ranging from 7 to 54 miles per day. From the 6th parallel of north latitude to the 10th parallel of south latitude the direction was again westerly, and the average velocity 35 miles per day, ranging from 17 to 70 miles per day. From thence to Tahiti the general tendency of the current was westerly, but its velocity was variable. The axis of greatest velocity of the counter-equatorial current was between the 7th and 8th parallels of north latitude. The axis of greatest velocity of the equatorial current was on the parallel of 2' north, where its speed amounted to 3 miles per hour."

The equatorial current strikes on the East Indian Archipelago, where it is split up by the narrow channels and shallow waters, and diverted into numberless minor currents. The two main divisions, which have acquired a high temperature from prolonged exposure to the tropical sun, ultimately leave the archipelago ; the southern arm curves southwards, carrying its warm water to the east coast of Australia and to New Zealand, whence it is diverted towards the east, and becomes merged again in the general north-easterly antarctic drift. The north equatorial current, which varies in volume and velocity with the monsoons, strikes the coast of Asia between the Philippines and Japan, and is deflected in a north-easterly direction as the Kuro-Siwo or Japan current - wholly a warm oceanic river during the S.E. monsoon similar to the Gulf Stream of the Atlantic. The Japan current sends many branches into the inland seas and channels of the north-eastern coast of Asia, but the main body of water flows northward until it bifurcates in 40° N. lat., sending one fork among the Kurile Islands and along the Kamchatka peninsula into Behring Sea, whence it escapes by Behring Strait into the Arctic Ocean. A small counter-current of arctic water flows southward through Behring Sea, but it is not of sufficient volume to make its influence felt very decidedly on the general temperature of the surface water in the vicinity. The second and larger branch of the Japan current crosses the North Pacific, and, curving southward by Alaska and British Columbia, part of it returns as the north equatorial current, while the rest forms the variable Mexican current that runs along the coasts of California and Mexico.

The general direction of surface circulation in the Pacific may be remembered by supposing the ocean divided. into a northern and southern half by the equatorial counter-current. In the northern half the water circulates in the direction of the hands of a watch, i.e., it passes up the west coast and down the east, while in the southern half the rotation is in the opposite direction - down the west coast and up the east; but the latter half does not exhibit the complete cycle so distinctly as the former. The centre of each area of circulation is occupied by a small Sargasso Sea, the northern being the more clearly defined, but neither approaches the well-known Sargasso Sea of the North Atlantic either in definiteness, extent, or amount of weed.

Temperature of Surface Water. - The distribution of temperature in the surface water of the Pacific varies eon-' siderably during the year. The equatorial region is of course comparatively little affected by the change of season, but there is a general rise of temperature in the northern parts of the ocean, and a fall in the southern, during the northern summer, and a similar rise in the south and fall in the north during winter. The charts exhibit a general northward move in the isothermals during the former season, and a southward tendency in the latter. The change in the position of the lines is greatest in the temperate zones. The charts of ocean surface temperature (see METEOROLOGY, figs. 8, 9) for February and August show the direction of the isothermals at two opposite seasons ; and reference to them will make it plain that in temperate regions the lines of equal temperature follow the parallels of latitude much more closely in the Pacific than in the Atlantic, while their displacement with the change of season takes place in a direction nearly north and south. There are notable instances of divergence from these rules, such as the peculiarity of the isothermal of 80° already alluded to. Another circumstance is the fact that the temperature of the surface water on the western side of a great continent is much lower titan that on the eastern side in the same latitude ; it seems as if the west side of a continent attracted the isothermals, making them converge towards the equator. It has already been pointed out that these effects are due to the winds and the cold currents which strike the western continental shores and run along the coasts. The surface temperature of the Pacific, between the latitudes of 45° N. and 45° S., nowhere at any season falls below 50°. In August the southern isotherm of 50° remains close to the 50th parallel, not diverging more than a degree or two on either side. Between the 45th parallels and the northern and southern limits of the ocean the temperature is almost always below 50°. The southern isotherm of 40° is remarkable for its constant position all the year round, between latitudes 55° and 58°, - a result brought about by the gigantic antarctic icebergs which prevent the surface temperature of the water from rising during the southern summer.

The northern and southern "isocrymes" of 68°, that is the lines which pass over water which has a mean temperature of 68° during the coldest months of the year, lie, according to Dana (Corals and Coral Islands, 1872), between the latitudes of 20° and 30° on each side of the equator, except in the neighbourhood of the South-American coast, where the isocryme runs north in a loop beyond the equator, - a consequence of the cooling effect of the Peruvian current. These isocrymes mark out an area of great importance ; for the reef-building corals are confined within it.

The highest temperature which sea water has been observed to attain is 90° F., and water of this temperature is only met with in the Red Sea. The maximum in the Pacific in the month of August is reached in the boundary between it and the Indian Ocean (in the Malay Archipelago) and in a narrow strip along the Mexican coast ; in both these regions the thermometer immersed in the surface water registers 85° as a mean. There is a considerable area which in August stretches between New Guinea and Japan, from 10° S. to nearly 30° N., where the surface temperature reaches 84', but these are exceptional temperatures.

When the " Challenger " was cruising in the South Pacific - in 1874 and 1875 - the water was found to be uniformly warmer than the air, the difference in temperature between the two averaging 1•5 to 2° Fahr. In the North Pacific, between the latitudes of 30° and 40°, on the other hand, the atmospheric temperature is about half a degree higher than that of the surface water. Such differences may be explained by considering the effect of warm and cold currents, which alter the temperature of the water much more rapidly than that of the air, and of warm and cold winds, which affect the atmosphere more quickly than the ocean.

ea Deep-Sea Temperature. - The serial temperature sounds- ings of the " Challenger " in the Pacific give a very good idea of the distribution of temperature in the deeper waters. There seems to be a slow massive movement of water from the Antarctic Ocean into the Pacific, which is not confined to the surface currents, but affects the whole mass of water down to the bottom. The rate of this motion is quite unknown. In the open sea, far from coasts and barriers, the temperature of the water continually decreases as the depth increases. This is only true for the open ocean, fully exposed to the effects of the mass movement of the water ; there is a very different distribution of temperature in enclosed seas such as those of the Western Pacific, or even in the ocean when a barrier presents itself to the moving water. The difference, which is I. brought out by the diagram (Plate II. fig. 1), is clue to the fact that when a barrier exists it retards the motion of the lower portion of the water, which has the lowest temperature, while the higher passes on over it, and fills up the area beyond with water at the uniform temperature of the great ocean at the point to which the top of the ridge or obstruction reaches. In the Sulu Sea, for instance, the diagram shows1 that the temperature falls steadily and rapidly from 80° at the surface to 50'5 at 400 fathoms, and then continues at 50°•5 right down to the bottom in 2500 fathoms, instead of sinking to somewhere about 35°, as it is observed to do in the open ocean at that depth. The inference is that the Sulu Sea is surrounded by a ridge rising to at least about 400 fathoms from the surface, which prevents the great ocean circulation from having its cooling effect, and soundings indicate that this is really the case. A study of the temperature phenomena, such as those just referred to, points out with considerable certainty the existence and height of barriers and ridges in many parts of the ocean, where their presence has not been detected by actual soundings.2 During the cruise of the " Challenger " the bottom temperature over the North Pacific was found to be 35'1; south of the Sandwich Islands it fell to 35°; in the Low Archipelago it again rose to 35'1; on the 40th parallel it fell to 34°• in the deep water, but rose to 35'4 and 35'5 in the shallow water of the Patagonian elevation.. The thermometer registered 34°-5 at the bottom between Australia and New Zealand ; while in that part of the ocean to the north-east of Australia known as the Coral Sea, although the depth was the same (about 2500 fathoms), the bottom temperature was as high as 35°.9. The variations of temperature in the enclosed seas of the Eastern Archipelago were found to be considerable, and nearly all those seas show the phenomenon of constant temperature from an intermediate point to the bottom, consequent on the existence of barriers. The chief details of the thermal conditions of these seas are represented. graphically in the diagram (Plate II. fig. 1). Between. the Caroline Islands and Japan the bottom temperature was 35°.3. The bottom temperature in the Pacific is on the average about 1° F. lower than that in the Atlantic.

The temperature of the water at the depth of 300 fathoms is nearly the same (40° to 45°) over the whole of the North Pacific, but above 300 fathoms the water is warmer in the western than in the central portion, while below that depth it is colder in the former than in the latter. The same phenomenon is noticed between the latitudes of 34° S. and 40° S., but here 700 fathoms marks the plane of constant temperature. Between 33° N. and 40° S. the temperature of the water above 200 fathoms is higher in the North than in the South Pacific, whilst from 200 to 1500 fathoms it is lower in the North, and below the latter depth the condition reverts to what it was above 200 fathoms.

The diagram (Plate II. fig. 2) exhibits the bathymetrieal distribution of temperature in a section of the Pacific from a 1 position in 38° 9' N. lat. and 156° 25' W. long. to one in 40° 3' S. lat. and 132° 58' W. long. as determined by H.M.S. " Challenger" in 1875, and may be compared with similar diagrams of the ATLANTIC (see vol. iii. p. 23). In order to separate the isothermals in the first 200 fathoms sufficiently the scale of depths required to be made large, while in order that the length of the diagram might be kept within reasonable bounds the scale of latitude was made very much smaller. The result of this is to exaggerate the inequalities of the sea bottom, making the slopes very much steeper than they are; this effect is best seen in the way in which islands are represented. The rapid falling off of temperature in the first few hundred fathoms, and then its very slow but steady decrease to the bottom are to be observed, and the fact that latitude has a great effect on the surface temperature, but none at considerable depths, for the isotherm of 40' is constantly between 300 and 900 fathoms, and also that depth alone determines the bottom temperature in the open ocean, the coldest water occurring as a matter of fact under the equator in the deepest troughs open to the south.

Density of the 1ra/cr. - The specific gravity of ocean water an index of its salinity, since the researches of various chemists, foremost amongst whom are Forchhammer and Dittmar, have shown conclusively that the percentage composition of the salts in sea water is the same in all parts of the ocean, so far at least as regards the principal constituents. Mr J. Y. Buchanan made continuous observations on the specific gravity of sea water during the whole voyage of the "Challenger," and has published a very valuable paper on the distribution of salt in the ocean in the "Challenger" Reports (Phys. Chem. Chill. Exp., • vol. i. part ii.). The chart in Plate II. showing the geographical distribution of surface density is copied from that paper. The percentage of total salts in sea water, as deduced from the specific gravity, is, according to Buchanan and Dittmar - Density 1.025 1 -026 1.027 1.028 Percentage.. 3 .3765 3.5049 3.6343 3'7637 The density of the water in different parts. of the ocean must obviously change to a certain extent with the season; and it is not only the surface density that is affected in this way ; any cause which promotes evaporation tends to increase the salinity of surface water, while any conditions that effect condensation of aqueous vapour produce dilution. For instance, in the China Sea during the month of November, at the end of the southwest monsoon, which is a moist wind accompanied by much rain, the specific gravity observed was 1:02518, and two months later, after the dry north-east monsoon had been blowing for some time, evaporation had proceeded so far that the specific gravity had risen to 1.02534. The climate is the principal factor in determining surface salinity, and the causes which produce well-marked climatic conditions have an equally apparent effect on the density of the water. Thus there are two zones of comparatively high density encirelthe globe in the region of tlA north-east and south-east tracde winds, which are dry and promote rapid evaporation ; and similarly the region of calms and rain between the trades is distinguished by the low specific gravity of the water. North and south of these areas there are two zones where the salinity maintains a mean value, in consequence of there being a balance between evaporation and condensation ; and round the poles there are areas of concentration brought about by the freezing of the sea water and the separation of salt, which of course increases the salinity of the water remaining unfrozen.

The distribution of density differs considerably in the two great oceans. In the Atlantic there are two areas of high- specific gravity, one in the north, the other in the south ; while in the Pacific there is only one, situated in the southern division of the ocean in the neighbourhood of the Society Islands. It is neither so large as those of the Atlantic, nor has it so high a specific gravity. The density of the concentration areas in the Atlantic, taking pure water at 4° C. as unity, is 1:02750 ; that in the saltest portion of the Pacific is only 1:02700. In the North Pacific the salinity is less than in the South, and its distribution is much more uniform. The density in this region never exceeds 1.02650, and the minimum, in the rainy region of the equatorial counter current, is as low as 1.02485. The South Pacific has water of a relatively high density, its maximum being 1.02750. The water of the seas of the Eastern Archipelago, in the western basin of the Pacific, although exposed to the full force of an equatorial sun, and possessed of a very high surface temperature, is yet surprisingly fresh. The specific gravity varies considerably with the season, but the average for the year over the greater part of these seas is under 1.02550 ; and there is a large area surrounding the islands of Java and Sumatra where the dilution is greater, the hydrometer only indicating 1.02500. The weak salinity of these waters is largely to be attributed to the extreme humidity of the atmosphere, the frequent and heavy rains, and the fact that so many lofty and extensive islands, where the annual rainfall rises above 200 inches, drain into the seas. Water of such a degree of dilution is not met with anywhere else, except near the mouths of rivers and in the vicinity of melting ice, and, as a temporary phenomenon, after prolonged rain in the tropics.

In regions where there is decided and continuous concentration in progress, the specific gravity of the water is greatest at the surface and decreases as the depth increases, down to about 800 or 1000 fathoms, after which the density increases slowly with the depth until the bottom is reached. The density of the bottom water of the Pacific is almost the same everywhere ; it only varies from 1'02570 to 1 02590; and the same value holds for the South Atlantic. The North Atlantic has denser water at the bottom, varying from 1.02616 to 1.02632. In those regions where the surface water is being constantly diluted, as is the case in the equatorial belt of calms, the density increases with the depth down to between 50 and 100 fathoms, where there is a maximum, from which the density diminishes, as in the other case, to about 1000 fathoms, and afterwards increases slowly clown to the bottom. There is a striking resemblance between the direction of the isohalsines, or lines of equal salinity, and of the isothermals ; but the parallelism breaks down, of course, in the case of a subsurface maximum.

Depth. - For a long time the opinion that the Pacific was a comparatively shallow ocean was entertained by geographers, and it is only the recent soundings of the " Challenger," " Tuscarora," " Gazelle," and other surveying ships that have succeeded in dispelling the illusion. It is now known that the average depth of the Pacific is greater than that of the Atlantic, and that areas of deeper water occur in it than in any other part of the globe. A line running along the western shores of the two Americas and along the eastern shores of the Asiatic continent more or less closely follows a great circle of the globe. On the one side of this line there are the continental masses of the Americas and of Europe and Asia, with an average height of about 800 feet above the level of the sea ; and on the other side the vast oceanic depression of the Pacific, with an average depth of about 2500 fathoms. The average level of continental area may thus be regarded as about three miles above the Pacific depression.

The attempt to divide the ocean into sharply defined basins is more or less unsatisfactory ; and for the consideration of the depth it is better to view the Pacific as marked off into two portions by an imaginary line passing through Honolulu and Tahiti, on the meridian of 150° W.

The eastern half is remarkable for the comparative absence of islands and the uniform nature of its depth. With the exception of the narrow strip of shallow water surrounding the Aleutian Islands and running along the American coast, the sounding line shows an average depth of from 2000 to 3000 fathoms undiversificd by remarkable elevations or depressions, between the northern limit of the ocean and 30' S. lat. There is a great submarine plateau extending from the Patagonian coast 76° W. long.) in a westerly direction to 120' W. long., which rises to between 2000 and 1000 fathoms of the surface. This elevated area diminishes in breadth as it proceeds westward, but it is supposed by some authorities to be connected with the shallow water surrounding the Low Archipelago and the Marquesas Islands (groups which are bisected by the 140th meridian of west longitude) and the S.Kiety Islands. If this be the case there is an almost continuous area of elevation stretching between Patagonia and Japan. It has been remarked that many of the submerged plateaus of the Pacific have a southeast to northwest trend. The " Challenger " examined the depth of the eastern half of the Pacific in 1875, along a line which extended fro n 33' N. lat. on the 160th meridian south-east to the Sandwich Islands, and then as nearly as possible along the 150th meridian to the Society Islands in 23° S. lat.. From this point the course was again south-east to the 40th parallel of south latitude, which was followed eastward to the Patagonian coast, a visit to Juan Fernandez forming a northward digression. The depth was ascertained at fifty points along this route, and it was found to vary on the whole from 2000 to 3000 fathoms. There were two soundings of over 3000 fathoms between latitudes 38° and 36° N., and one a little to the south of the Sandwich Islands. Between the meridian of 120° W. and the coast of America the soundings showed the depth to vary considerably as the ship was in deep water or over the submerged Patagonian plateau. The actual numbers observed proceeding eastward from 120° W. long. were in fathoms :-2250, 1600, 2025, 2270, 1500, 1825, 1775, 1375, 2160, 2225, 1150, 1325. The soundings made by the United States ship " Tuscarora " during 1874 were much more numerous, closer together, and extended along several lines, but the general result was similar to that of the " Challenger " observations. The results of all recent observations are shown on Plate III.

The western half of the Pacific Ocean is a complete contrast to the eastern. Archipelagos and scattered islands are exceedingly numerous ; the depth of the ocean is by no means uniform, for shallows and areas of unusual depth occur scattered over it at irregular intervals. Along the Asiatic coast and between the island groups there are a number of partially enclosed seas, and these are separated from the great ocean by submarine plateaus of sufficient extent and height to warrant the supposition that a moderate upheaval would extend the Asiatic continent as far south as Australia, transforming the seas into inland salt lakes. Considerations of the peculiar animal and vegetable life of New Zealand and Australia lend some degree of probability to the speculation that these islands were joined to the main continent of Asia at some very remote period ; and it is even possible to trace the submerged coast-line of the great continent which then existed. This line separates the very deep water of the West Pacific from the shallower water of the inland seas and archipelagos ; it runs from Kamchatka, over Japan, Formosa, the Philippines, New Guinea, to Australia and New Zealand. The most conspicuous peculiarity of the West Pacific is the very deep water lying in a crescent shape to the east of the Kurile Islands and Japan. It extends from 50° N. lat. to nearly 20' N. lat., although it is of no great breadth. The average depth of this area is nearly 4000 fathoms, and a narrow strip of still more abysmal depths runs along its western margin, like a ditch across the entrance to the Sea of Okhotsk ; here the United States ship "Tuscarora " found depths of over 4600 fathoms. The course of the " Challenger " led her to explore the seas of the Eastern Archipelago pretty thoroughly, and she carried a line of soundings from the archipelago to Japan, and thence eastward across the Pacific, crossing the area of great depth about the centre, off Nippon, where two soundings of 3950 and 3525 fathoms respectively were obtained. Like the East Pacific, the western division of the ocean has an average depth of from 2000 to 3000 fathoms, although a great number of small depressions exist where the depth is greater, and detached areas of shallower water occur still more frequently. Many of the islands rise from depths of about 3000 fathoms, forming isolated mountains springing from the bed of the ocean, and several peaks which do not rise to the surface have been detected. More usually a number of islands are bound together by submarine elevations, frequently within a few hundred fathoms of the surface, over wide areas. Although the greater part of the sea surrounding New Zealand, the north of Australia, and the adjacent islands is under 1000 fathoms in depth, there are areas of great depression amongst the islands, and some very deep channels. In 1875 when sounding in the channel between the Carolines and Ladrones, the " Challenger " met with the deepest water of the cruise, 4475 fathoms, or about five miles and a quarter ; and this is the greatest depth from which a specimen of the bottom has hitherto been obtained. This abysmal depth only extends over a relatively small area, for the two nearest " Challenger " stations, one to the north and one to the south, had depths of 2300 and 1850 fathoms respectively.

The seas branching off from the Pacific are usually relatively shallow. Bebring Sea on the north has extremely Ahallow water in its north-eastern half, where there is a depth of under 100 fathoms ; in the south-western portion the depth increases rapidly to between 1000 and 2000 fathoms, except round the coasts and the Aleutian Archipelago. The Sea of Okhotsk is still shallower: much of it is within the 100 fathom line ; and in its deepest part it does not attain 1000 fathoms. The Yellow Sea is entirely within the hundred fathom line ; while the Sea of Japan, only separated from it by the Corean Peninsula, is not inferior in depth to the open ocean, its average depth being from 2000 to 3000 fathoms. The western portion of the Pacific, which lies between the Philippines and the Carolines and Ladrones, is also very deep, its mean depth approaching 3000 fathoms. This sea is of importance, since it is to the Pacific what the Gulf of Mexico is to the Atlantic - the source of its great northern thermal current. The fact that the temperature at 1500 fathoms over the whole of the North Pacific does not differ by more than 0'•5 F. from that at the bottom appears to indicate that this portion is cut off from the southern division by a ridge rising to within 1500 fathoms of the surface. The existence of such a barrier cannot be said to be proved, but the indications lead to the supposition that it may extend from Japan to the equator, through the Bonin, the Ladrone, and the Caroline Islands.

Taken altogether, so far as present knowledge goes, the bed of the Pacific is more uniform than that of the Atlantic, and its changes of level are less abrupt. Its depth is, on an average, greater, and appears to be more evenly distributed than in the Atlantic, but this apparent greater uniformity may be partly due to the fact that the latter ocean, both on account of its smaller size and its greater commercial importance, has been much more carefully surveyed, and its bathymetrical conditions more exactly ascertained.

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